The present invention concerns a device for adjusting and/or testing the axial tightening forces in screw joints.
The invention concerns above all mechanical screw joints which are used with different designs and in many ways in the field of mechanics. Basically, it can be applied to all known kinds of threads or screw joints.
Devices for adjusting the axial force in screw joints, including a check device for limiting an axial force operating between e.g. two force-applying elements of the screw joint, are already known. Thus so-called “torque wrenches” which allow mechanical overload control upon exceeding a given torque for a screw joint, are known. In this way, precise detection of the force between the force-applying elements (e.g. a bolt head on a bolt shank as the first force-applying element and a nut on a thread of the bolt shank as the second force-applying element) is not definitely possible. The torque which is applied to tighten a screw joint does cause a change in the axial force between the force-applying elements, as a rotational movement produces a change of distance between the force-applying elements and hence a change in axial force, due to the pitch of the thread of a bolt shank. However, the correlation between axial force and torque is under certain circumstances highly subject to errors, as the frictional forces of a force-applying element on a thread influence the torque critically. When the torques are identical, the tightening force of a screw joint can vary considerably, so that for bolts which slide easily there is a risk of overstretching, whereas in the case of stiff bolts the preselected torque is already attained when the parts to be connected are still loose. Apart from this imprecision, with the mechanical overload protection there is the drawback that only exceeding of the limit is possible, but not exact measurement of the axial force.